The more I think about the Shor/McCarty graph showing legislative partisanship in the states, the more concerns I have. Don't get me wrong -- it's an amazing graph with a ton of important data behind it. But I'm increasingly concerned about the hidden role of agenda control.
The graph below is a density plot of the percent of the chamber voting yes on all the bills that came before it in the 1999-2000 legislative session. It charts this distribution for both the California and Florida assemblies. (The data come from the Representation in America's Legislatures project.)What do we see here? In Florida, the distribution is bimodal. Most pieces of legislation either get around 40% of the vote or around 80% of the vote. Why would so many bills get 40% of the chamber voting yes? They are minority party bills. The minority party usually loses, but they can still get their bills to a floor vote.
Contrast that with the distribution for California. Again, the distribution is bimodal, but the peaks are around 60% of the chamber (a narrow win) and around 90% (near unanimity). Almost no bills get below 50% of the chamber voting yes. In other words, the minority party's bills almost never make it to the chamber floor. The majority party makes sure that they die in committee.
This can have a substantial effect on the appearance of party polarization. In California, bills that might otherwise give the chamber more of an appearance of bipartisanship are being killed by the majority party before they can reach a floor vote. This may account for California having the most divergent parties in the Shor/McCarty graph. Or maybe it really is due to the legislators being so ideologically dissimilar. It's hard to tell, and I don't know of any multi-year measures of state-level agenda control that would allow us to factor this effect out.
One other issue. As some of the commenters at 538 noted, it's kind of weird to see California's GOP further to the right than any of the southern states' Republican parties. Now this is certainly possible. The Shor/McCarty data compresses everything onto one dimension. I'm guessing that the southern Republicans would look more conservative on cultural measures than the CA GOP, although the Californians are probably more conservative on economic measures. But I'm not sure.
Update: Boris Shor responds.
Later update: Boris is probably right that agenda control isn't biasing these results all that much. As he notes, the U.S. House and Senate have very different agenda control rules, yet members show remarkably consistency in their ideal points when they move from one chamber to the other. However, I don't know whether the differences between the House and the Senate are on par with the differences across state legislatures. The differences between Florida and California strike me as pretty huge.